The present invention generally relates to multidimensional chromatography, and particularly to an on-line coupled LC/GC system which is capable of generating information for compounds containing nonvolatile components.
Multidimensional chromatography can be a powerful separation tool, especially when dealing with complex matrices which may require unattainably high theoretical plate counts for adequate resolution. Similarly, multidimensional chromatography has been found quite useful when dealing with samples that require tedious clean-up steps prior to analysis. The combination of a liquid chromatograph (LC) and a gas chromatograph (GC) in an "on-line" mode has been described in the following references: "On-Line Multidimensional Chromatography Using Packed Capillary Liquid Chromatography And Capillary Gas Chromatography", by H. J. Cortes, C. D. Pfeiffer, B. E. Richter, in HRC & CC, 8 (1985) 469., "Determination Of Trace Chlorinated Benzenes In Fuel Oil By On-Line Multidimensional Chromatography Using packed Capillary Liquid Chromatography And Capillary Gas Chromatography", by H. J. Cortes, C. D. Pfeiffer, B. E. Richter, D. E. Jensen, in J. Chromatogr., 349 (1985) 55; and "On-Line Multidimensional Chromatography Using Micro HPLC-Capillary GC", by H. J. Cortes, C. D. Pfeiffer, in Chromatography Forum, 4 (1986) 29. These articles are incorporated herein by reference.
As discussed in these articles, columns for High Performance Liquid Chromatography (HPLC) have been used in an on-line mode primarily for the determination of trace components in complex matrices, where the LC provides a highly efficient clean-up step, and a section of the chromatogram containing the components of interest is transferred to a GC for further resolution and quantification. A similar system has also been used to separate components by class, as discussed in "Coupling Micro LC-Capillary GC As A Powerful Tool For The Analysis Of Complex Mixtures", by D. Duquet, C. Dewaele, M. Verzele, in HRC & CC, 11 (1988) 252.
Nevertheless, a principal limitation to the use of this LC/GC technology is the type of compounds that can be analyzed by the GC. In other words, the compounds must be volatile and chromatographable in the gas phase. Nonvolatile or highly polar compounds can be analyzed by a GC if they are chemically treated (derivatized) to convert them into a more suitable form. In this regard, see the "Handbook of Derivatives For Chromatography", by K. Blau, and G. King, Heyden & Son, Ltd., London (1978). However, the need to chemically treat these compounds makes it very difficult to provide an on-line or uninterrupted multidimensional analysis of such nonvolatile or highly polar compounds.
Another alternative is the use of pyrolysis gas chromatography to examine the volatile pyrolysis fragments of a nonvolatile molecule. In the characterization of polymers, the combination of size-exclusion chromatography (SEC) and pyrolysis gas chromatography will enable the determination of average polymer composition as a function of molecular size/weight. However, this type of information is difficult to obtain, since fractions eluting from an SEC system are usually collected manually, evaporated, redissolved in an appropriate solvent and manually transferred to a pyrolysis probe via a syringe.
Accordingly, it is a principal objective of the present invention to provide a system for coupling liquid and gas chromatography which will permit on-line multidimensional analysis or determinations of nonvolatile or highly polar compounds. In this regard, the term "nonvolatile" will be used herein to refer to compounds having nonvolatile and/or highly polar characteristics.
It is another objective of the present invention to combine size-exclusion chromatography (SEC) and pyrolysis gas chromatography in an on-line system to permit the determination of the average polymer composition as a function of molecular size/weight, as well as to provide valuable information which can be used to understand polymer properties and polymerization chemistry.
It is a further objective of the present invention to provide an on-line multidimensional system which is capable of automatically collecting fractions of interest from an SEC and transferring them to an interface for permitting GC analysis.
To achieve the foregoing objectives, the present invention provides an on-line multidimensional system which includes a micro size-exclusion chromatograph, and a switching valve for sampling fractions eluting from the SEC. The switching valve directly transfers these sampled fractions to a pyrolysis probe, which produces volatile fragments representative of the nonvolatile components from the sampled fractions. The system also includes a gas chromatograph for providing molecular size/weight distribution information from the volatile fragments. The present invention also enables the combination of the valve, pyrolysis probe and gas chromatograph to be used as an autoinjector when the SEC or other LC is not connected to the system.
The switching valve combines separate sample and solvent flush loops, and causes a carrier fluid to sequentially convey the contents of these loops to the pyrolysis probe. The pyrolysis probe includes a pyrolysis ribbon which is coaxially disposed within a glass housing. The housing for the pyrolysis probe includes a lateral well portion which directs the sampled fractions to a confined area of the pyrolysis ribbon. The pyrolysis probe housing also permits the introduction of an auxiliary carrier fluid to increase the solvent evaporation rate and minimize the opportunity for the solvent to spread along the pyrolysis ribbon.
Additional advantages and features of the present invention will become apparent from a reading of the detailed description of the preferred embodiments which makes reference to the following set of drawings in which: